The photoconduction process using electrophotographic light-sensitive materials requires
(1) a step of generating electric charges by exposure, and PA0 (2) a step of transferring electric charges.
Steps (1) and (2) can be conducted using the same substance, such as a selenium light-sensitive plate. Alternatively, steps (1) and (2) can be performed using different substances, such as a combination of amorphous selenium and poly-N-vinylcarbazole, respectively. Using different substances in steps (1) and (2) has the advantage that the selection of substances used for electrophotographic light-sensitive materials is widely extended and, consequently, the electrophotographic characteristics of the electrophotographic light-sensitive materials, such as sensitivity or acceptive electric potential, can be improved.
Conventional substances used as the photoconductive raw materials for electrophotographic light-sensitive materials used for electrophotography include inorganic substances such as selenium, cadmium sulfide or zinc oxide.
The electrophotographic process as disclosed by Carlson in U.S. Pat. No. 2,297,691 uses a photoconductive material comprising a base coated with a substance that is electrically insulative before exposure to light, having an electric resistance which changes according to the amount of imagewise exposure to light. This photoconductive material is generally uniformly charged to form a surface electric charge after being subjected to dark adaptation for a suitable time. Then it is imagewise exposed to light having the effect of reducing the surface electric charges according to the relative energy of the various parts of the image exposed. The reisdual surface electric charge or electrostatic latent image on the surface of the photoconductive substance layer (electrophotographic light-sensitive layer) is then brought into contact with a suitable electroscopic indicating substance, namely, a toner to form visible images.
The toner is contained in an insulating liquid or a dried carrier. In any case, it can be adhered to the surface of the electrophotographic light-sensitive layer according to the electric charge pattern. The adhered indicating substance can be fixed by known means such as heat, pressure or vapor of a solvent. Further, the electrostatic latent image can be transferred to a second support (for example, paper or film, etc.). In addition, the electrostatic latent images can be developed on the second support after being transferred. The electrophotographic process is an image forming process in which images are formed as described above.
In such an electrophotographic process, the fundamental characteristics required for electrophotographic light-sensitive materials are (1) that they can be electrically charged in the dark to have a suitable electric potential, (2) that the rate of discharge of the electric charges in the dark is low, and (3) that the electric charges can be discharged quickly by exposing to light.
The above-described conventional inorganic substances have many advantages but they have various disadvantages. For example, selenium, which is widely used at present, satisfies the above requirements (1) to (3), but it has the disadvantage that its production is difficult aand involves increased production costs, it is lacking in flexibility and can only be formed into belt shape with difficulty, and it must be handled carefully, because it is very sensitive to heat or mechanical impact. Cadmium sulfide and zinc oxide have been used as electrophotographic light-sensitive materials dispersed in a resin binder. However, they cannot be used repeatedly because of mechanical faults in smoothness, hardness, tensile strength or friction resistance, etc.
In recent years, in order to overcome the disadvantage of such inorganic substances, electrophotographic light-sensitive materials using organic substances have been developed and some of them have been put into practical use, including electrophotographic light-sensitive materials composed of poly-N-vinylcarbazole and 2,4,7-trinitrofluoroen-9-one (U.S. Pat. No. 3,484,237), those having poly-N-vinylcarbazole sensitized by pyrylium dyes (Japanese Patent Publication No. 25658/73), electrophotographic light-sensitive materials containing an organic pigment as a chief component (Japanese Patent Application (OPI) No. 37543/72 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")) and electrophotographic light-sensitive materials containing eutectic complex composed of a dye and a resin as a chief component (Japanese Patent Application (OPI) No. 10735/72).
Although the mechanical characteristics and flexibility of these organic electrophotographic light-sensitive materials can be improved to some extent over those of inorganic electrophotographic light-sensitive materials, they have low light sensitivity and are not suitable for repeated use. Accordingly, they are insufficiently practical for use as electrophotographic light-sensitive materials.